226 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
larger crustaceans, insect larvae (especially caseworms), and other alongshore material, 
together with the dead and empty debris of insect transformation. It was in fact 
very clear that the frequently observed basking of small fishes in these warm waters 
was not caused by a greater abundance of their food. 
A comparison of the collections made at and beneath the surface, by day and by 
night, in sunshiny and in cloudy weather, would seem to indicate that the lake variety 
of Daphnia pulex is much more sensitive to sunlight than any other associated form. 
In collections made at the surface after dark, and in those made in sunny weather 
below the surface, this was many times the most abundant crustacean; but in similar 
collections made at the surface in sunshine it was relatively rare, Diaptomus sicilis 
then taking the lead. 
Only brief time and scanty opportunity could be had for miscellaneous alongshore 
work during our visits to Yellowstone Lake. The greater importance, from our point 
of view, of deep-water work, the stormy weather, and the unfavorable character of the 
beaches, commonly either covered with bare gravel or packed with large boulders and 
beaten by surf, made such work unprofitable and difficult. Nearly all our knowledge 
of this alongshore fauna we owe, in fact, to a day spent in 1891 on Stevenson Island 
(1^ miles long by ^ mile wide), lying 2 miles from the hotel landing, in the north 
end of the lake. The shores of this island vary from precipitous bluffs on the west to 
a weedy shallow on the south. The beach is gravelly everywhere, except as boulders 
thickly cover the bottom and banks, and outside this is a belt of gravelly sand, fol- 
lowed by sandy mud still farther out. Water weeds occur in scanty patches, chiefly 
Chara and algae; a coating of minute, dirty algae commonly covers the stones. On the 
stones are also countless tubes of small Cliironomus larvae, almost covering the sur- 
face, mostly emptied by the maturing of these insects at the time of our visit. Under 
the stones are a considerable number of leeches of various sorts (mostly Nephelis and 
Clepsine ), and an occasional small annelid worm. On them, among the weeds, a small, 
black, spiral mollusk, Physa , may occur by hundreds, and creeping under or over them, 
or swimming through the water just above the bottom, we found an occasional Gam- 
mams. A large species of Gorisa , several water-beetles ( Deronectes ), and a perlid 
larva were all the other insect forms taken here. A small collection of entomostraca 
from the weeds has not been determined. 
Our knowledge of the bottom fauna of Yellowstone Lake is based on the product 
of eleven dredgings, at depths varying from 15 to 198 feet. Four of these dredgings 
were in shallow water, 20 feet or less ; two were from a medium depth (25 to 50 feet) ; one 
ranged from shallow water to deep (20 to 120 feet); one from a medium to a consider- 
able depth (40 to 100 feet) ; and three may be classed as deep throughout, ranging 
from 186 to 198 feet. In this deepest water the most abundant inhabitants of the 
bottom were long and slender annelid worms (Oligochceta) not yet studied, blood-red 
larvie of Chironomus of considerable size, and a small bivalve mollusk ( Pisidium ). 
Several leeches also occurred in our deepest hauls ( Nephelis maculata and species of 
Clepsine ), a few specimens of Gammarus , probably taken after the dredge left the 
bottom, several small planarians, a lot of living Spongilla , and in one instance a Physa on 
living vegetation — in all probability a floating fragment, as the haul on the bottom 
here was made at not less than 186 feet. In other dredgings, at 100 feet or more, many 
specimens of Gypris , a very few hydrachuids (probably swimming above the bottom), 
and several worms (nematoids) were added to the above list. 
